Method for testing the integrity of a flexible tubular pipe and device for implementing same

ABSTRACT

Method for testing a pipe for carrying hydrocarbons. The pipe has at least one internal sealing sheath made of polymer material, incorporating elements of reactive compound capable of reacting with corrosive gases contained in the hydrocarbons which diffuse radially through the sheath. The reaction forms a first layer, extending radially from the internal surface, in which the elements of reactive compound have reacted with the gases. A second layer, extends between the first layer and the external surface, in which the elements of reactive compound have not yet reacted with the gases. The method uses ultrasound to determine the position of an interface between the first and second layers to measure the progression of the diffusion of the gases through the sheath.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a divisional of prior U.S. patentapplication Ser. No. 13/808,831, filed Jan. 31, 2013, by Marie-HeleneKlopffer, Xavier Lefebvre, Yann Nicolas and Patrice Jung, entitled“METHOD FOR TESTING THE INTEGRITY OF A FLEXIBLE TUBULAR PIPE AND DEVICEFOR IMPLEMENTING SAME,” the entire contents of which are incorporatedherein by reference. U.S. patent application Ser. No. 13/808,831 is a 35U.S.C. §371 National Phase conversion of International Application No.PCT/FR2011/051574, filed Jul. 5, 2011, which claims priority of FrenchPatent Application No. 1055557, filed Jul. 8, 2010, the entire contentsof which are incorporated herein by reference.

The present invention relates to the field of monitoring tubular pipesespecially intended to be used by the offshore drilling industry, forexample to transport hydrocarbons. It more particularly relates to amethod used to inspect the integrity of such a pipe with respect to theaction of aggressive fluids, especially corrosive gases, for example H₂Sand CO₂ present in the hydrocarbons transported by the pipe.

The invention especially targets flexible submarine pipes intended totransport hydrocarbons the pressure and temperature of which may reach1000 bar and 130° C., respectively. Such a flexible pipe, in its generalform, is well known in the art and takes various configurationsdepending on its exact use but generally meets the structural criteriaespecially defined in standards API 17 RP B and API 17J established bythe American Petroleum Institute under the titles “Recommended Practicefor Flexible Pipe” and “Specification for Unbonded Flexible Pipe”.

A common type of such flexible oil and gas pipes comprises, from theinterior to the exterior: an internal carcass of interlocking metalstrips intended to prevent the pipe from being crushed; an internalsheath made of an extrudable thermoplastic, generally a polymer, bearingagainst the carcass and intended to provide the pipe with an internalseal; a set of metal armor layers intended to withstand compressiveand/or tensile stresses; and an external sheath that protects and seals,made of a thermoplastic, generally a polymer.

An inherent problem with the transportation of hydrocarbons in thesepipes, in particular at high temperatures and pressures, is related tothe permeability of the internal sheath to corrosive fluids.Specifically, gases such as H₂S are able to diffuse through thethermoplastics used to fabricate the sheaths, and it is known that theflow rate of gas through thermoplastics increases with temperature andpressure. Now, H₂S is a gas that corrodes unalloyed or low-alloy carbonsteels from which the various metal armor layers present in the annularcavity, located between the two, internal and external, sheaths, in thepipe, are usually made. Therefore, when such corrosive gases diffusethrough the internal sheath and penetrate into the annular cavitylocated between the internal and external sheaths, the metal armorlayers of the pipe may, under certain conditions, undergo corrosionwhich may eventually have a critical effect on the integrity of thepipe.

One of the solutions envisioned to solve this problem of corrosionconsists in placing intermediate between the hydrocarbon flowing throughthe pipe and the metal armor layers a sheath made of a polymerincorporating a reactive compound, the latter being dispersed throughsaid polymer, this reactive compound being able to react with the acidicgases in order to neutralize them. In this way, the sheath is sealedwith respect to the gases in question.

Document EP 0 844 429 describes such a solution. The sealing barrierlayer is an extruded sheath made of polyethylene filled with fineparticles of zinc oxide (ZnO). This reactive ZnO compound reacts withH₂S so as to neutralize it, forming ZnS, which remains trapped in thesheath, and water (H₂O) that diffuses through the sheath. This solutionis very effective provided that some reactive compound remains in thebarrier layer. However, it has the drawback of having a limitedlifetime, since it loses its effectiveness once all of the reactive ZnOcompound has reacted with H₂S. From this moment on, H₂S can freelydiffuse through the barrier layer and reach the metal armor layers ofthe flexible pipe. Corrosion of the metal armor layers may then greatlyaccelerate, thereby risking a significant reduction in the remaininglifetime of the pipe. Specifically, the metal armor layers of thesepipes are not designed to withstand, for any length of time, the rate ofcorrosion that they would have to endure in the absence of the barrierlayer; hence, if this barrier layer abnormally and prematurely loses itsseal, the remaining lifetime of the pipe would be substantiallyshortened.

Although the lifetime of such a barrier layer can be evaluated using anexperimentally validated theoretical physical model, it is neverthelessdesirable to instrument the flexible pipe with means not only allowingthe correct operation of the barrier layer to be checked in real time,but also allowing its remaining lifetime to be accurately predicted.This is because replacing a pipe is a very complex and expensiveoperation which requires production to be stopped and which is liable tolead to very large additional costs if not correctly predicted andplanned for.

A method for monitoring a pipe of the aforementioned type, based on ananalysis, using spectroscopic techniques, of the chemical composition ofthe fluids present in the annular cavity between the internal andexternal sheaths of the pipe, is known from document WO 2009/106078. Themethod described in this document thus makes it possible to effectivelydetect the presence of corrosive gases coming from the hydrocarbonstransported by the pipe and having penetrated into the annular cavitythrough the internal sheath, by means of which a threat to the safety ofthe pipe may rapidly be diagnosed.

However, this method for monitoring a pipe is not completelysatisfactory in that, although it allows, by delivering informationregarding the presence of corrosive gases in the annular cavity, animmediate risk of accelerated corrosion of the metal armors layer of thepipe to be diagnosed, it does not, in contrast, allow the onset of thisproblem to be predicted. Specifically, since the measurement is carriedout in the annular cavity, when corrosive gases are detected they arealready in contact with the metal armor layers and corrosion maytherefore, under certain conditions, have already started.

In this context, the aim of the present invention is to provide a methodand a device for monitoring a pipe of the aforementioned type, meetingthis need and which may provide an early indication of the remaininglifetime of the pipe.

The invention achieves this aim by providing a method for monitoring atubular pipe intended to transport hydrocarbons containing corrosivegases, said pipe comprising at least one internal polymer sheath intowhich said corrosive gases are liable to diffuse radially from an innersurface to an outer surface of said internal sheath via a radialdiffusion effect, said polymer of said internal sheath incorporatingreactive-compound elements dispersed through the thickness of saidinternal sheath and able to react with said corrosive gases in order toneutralize them via a neutralizing reaction, said neutralizing reactionand said radial diffusion effect forming, on the one hand, a firstlayer, in said internal sheath, in which said reactive-compound elementshave reacted with said corrosive gases, said first layer graduallyextending through the thickness of said internal sheath from said innersurface and, on the other hand, a second layer occupying the residualthickness of said internal sheath, in which layer said reactive-compoundelements have still not reacted with said corrosive gases, said secondlayer extending between said first layer and said outer surface of saidinternal sheath, noteworthy in that the position of an interface betweensaid first layer and said second layer occupying some thickness of saidinternal sheath is determined using ultrasound, so as to measure, inreal time, the progression of the diffusion of said corrosive gasesthrough the thickness of said internal sheath.

When ultrasonic waves meet an interface bounding two media havingdifferent acoustic impedances, the waves are reflected giving rise to anecho according to the principle of ultrasonography, well known inmedical imaging for example.

By applying this principle, the method of the invention allows theposition of an interface representing the advance of a front ofconversion of the reactive-compound elements dispersed through thethickness of the sheath, after a chemical reaction with the corrosivegases gradually diffusing through the thickness of the sheath, to belocated, the advance of this front of conversion of thesereactive-compound elements then serving as a marker for quantifying theadvance of the corrosive gases through the thickness of the sheath.Specifically, this conversion front, which is located at the innersurface of the internal sheath at the start of the life of the pipe,then progresses over time through the thickness of the sheath, in orderto finish at its outer surface when all the reactive-compound elementshave been consumed via chemical reaction with the corrosive gases,advantageously bounds two layers occupying some thickness of theinternal sheath, respectively a first layer extending gradually throughthe thickness of the sheath from its inner surface and consisting of amixture of the polymer and the products of the chemical reaction betweenthe reactive-compound elements and the corrosive gases, and a secondlayer occupying a residual thickness, extending between the first layerand the outer surface of the sheath and consisting of a mixture of thepolymer and the intact reactive-compound elements that have not yet beenreached by the corrosive gases.

The use of the ultrasonography principle recalled above to detect andlocate the interface between these two layers occupying graduallychanging thicknesses in the internal sheath, will, prima facie, be ofgreat surprise to those skilled in the art.

This is because the materials of these two layers occupying somethickness have mechanical properties that are very similar to those ofthe polymer forming the sheath, and therefore there is very littledifference between them, on account of the small amount of the productsproduced by the chemical reaction and of the reactive-compound elementspresent, respectively, in the first and second layers. It is a prioriclear that, with mechanical properties that are so similar, it wouldnot, based on common knowledge, be possible to envision that atransition between the materials forming these two layers occupying somethickness in the sheath would generate an ultrasound echo allowing theposition of any interface between these two layers to be detected andlocated, and a person skilled in the art would therefore discard theidea of using an ultrasonography method for this purpose.

According to the invention, it has been discovered that the chemicalreaction between reactive-compound elements dispersed through thethickness of the sheath and corrosive gases diffusing through thesheath, forming a first layer that extends gradually through thethickness of the sheath from its inner surface, is nevertheless enoughto sufficiently modify the mechanical properties of this first layer ofthe sheath, relative to those of the second layer occupying a residualthickness in the sheath and extending between this first layer and theouter surface of the sheath, that it is indeed possible to use anultrasound monitoring method to detect and locate the interface betweenthese two layers and thus to quantify, in real time, the advance of thefront of conversion of the reactive-compound elements, revealing theadvance of the diffusion of corrosive gases through the thickness of thesheath, by means of which it is possible to predict the remaininglifetime of the pipe.

To do this, an ultrasound beam is emitted, from the outer side of theinternal sheath in the direction of the interior of the pipe, so thatthe ultrasound beam passes through the thickness of the internal sheathbetween its outer surface and its inner surface, the ultrasonic wavesreflected in the thickness of said sheath by said interface arecollected, in the form of signals, and said signals are processed so asto determine the position of said interface in the thickness of saidinternal sheath.

Advantageously, the central frequency of the ultrasound beam liesbetween 1 and 5 MHz, preferably between 1.5 and 3 MHz, advantageouslybetween 2 and 2.5 MHz, and more advantageously is about 2.25 MHz.Advantageously, the pulse width, measured at −20 dB, lies between 0.5and 1.5 μs and is advantageously smaller than 1 μs. Advantageously, thebandwidth, measured at −6 dB, lies between 1 and 4 MHz, and a spectrumis used comprising a sufficiently large number of waves with a lowfrequency of about 1 MHz the amplitude difference of which, relative tothe central frequency, must not be less than 30 dB.

Again advantageously, the shape of the ultrasound beam is focused andthe beam focal point is positioned toward or beyond the bottom of saidinternal sheath.

The ultrasound transducer is a piezoelectric transducer and ispreferably a piezocomposite transducer. In this case it is advantageousfor a focus to be achieved by shaping the piezocomposite componentitself.

Advantageously, a square-shaped or annular two-dimensional array ofelementary transducers is used. In this case, the ultrasound beam ispreferably controlled using an electronic phase-shift technique.

Furthermore, advantageously, the reactive compound is chosen from ZnO,PbO, CuO, CdO, NiO, SnO₂ and MoO₃.

According to a first embodiment of the invention, the tubular pipe maybe a flexible pipe. The term “internal sheath” must then be understoodwith a wide sense as denoting any type of internal sheath of theflexible pipe. Specifically, the reactive-compound elements may beincorporated either into the first sheath starting from the interior ofthe pipe, or into an intermediate sheath located between, on the onehand, this first sheath and, on the other hand, the external sheath.

According to a second embodiment of the invention, the tubular pipe maybe a rigid pipe, and especially a rigid pipe comprising an internalpolymer sheath surrounded by a metal tube. The polymer sheath has thefunction of lining the interior of the metal pipe in order to protect itfrom corrosion. This type of pipe is especially described in document WO00/77587. The protection provided by the internal polymer sheath orliner may be improved by incorporating a reactive agent into the polymerused to form said liner. In this case, the present invention allows theadvance of the diffusion of acidic gases into the liner to be monitoredand the moment when these gases will reach the inner wall of the metaltube to be predicted.

The invention also relates to a monitoring section able to be attachedto a tubular pipe intended to transport hydrocarbons containingcorrosive gases, noteworthy in that it comprises, from the interior tothe exterior, a number of coaxial layers and especially at least oneinternal sheath and a cylindrical metal cover fitted around saidinternal sheath, said internal sheath being made of a polymerincorporating reactive-compound elements dispersed through the thicknessof said sheath and able to react with corrosive gases in order toneutralize them, said corrosive gases being liable to diffuse radiallyfrom an inner surface toward an outer surface of said internal sheath,thereby forming, on the one hand, a first layer in which saidreactive-compound elements have reacted with said corrosive gases, saidfirst layer gradually extending through the thickness of said sheathfrom said inner surface and, on the other hand, a second layer occupyingthe residual thickness of said sheath, in which second layer saidreactive-compound elements have still not reacted with said corrosivegases, said second layer extending between said first layer and saidouter surface of said sheath, said monitoring section comprising anintegrated ultrasonic transducer able to determine the position of aninterface between said first layer and said second layer occupying somethickness of said sheath using ultrasound so as to measure, in realtime, the progression of the diffusion of said corrosive gases throughthe thickness of said internal sheath.

It is difficult to integrate an ultrasound sensor in an oil and gas pipeof great length, particularly when the latter is a flexible pipecomprising many layers. It is thus simpler to produce a shortinstrumented section, taking care to ensure that this section has astructure representative of the main pipe as regards diffusion andneutralization of the acidic gases, and to then connect this section inseries with the main, flexible or rigid, pipe so as to form atransportation pipe according to the present invention.

Preferably, the ultrasonic transducer is housed in the cylindrical metalcover, the residual thickness of the cover left between the front sideof the transducer and the outer surface of the internal sheath beinglarger than at least three times the thickness of said sheath.

Advantageously, first means are provided for coupling the front side ofthe ultrasonic transducer to the cylindrical metal cover and secondmeans for coupling the outer surface of the internal sheath to thecylindrical metal cover.

Advantageously, the second coupling means comprise pressurized means formechanically coupling the outer surface of the internal sheath to aninner surface of the cylindrical metal cover.

The cylindrical metal cover may comprise a circuit for draining gasesdiffusing through the internal sheath, said draining circuit comprisinga set of grooves formed on an inner surface of said cover at aninterface with an outer surface of the internal sheath, said set ofgrooves being connected to an aperture able to be closed by a removableblocking means and communicating with the exterior of said monitoringsection, by means of which the pressure to which said internal sheath issubjected may be controlled.

Advantageously, the monitoring section comprises two connecting flangesrespectively fitted at either end of said cylindrical metal cover and onwhich flanges crimping cones are arranged to bear against said internalsheath.

The invention also relates to a pipe intended to transport hydrocarbonscontaining corrosive gases, said pipe comprising at least one monitoringsection of the invention, noteworthy in that said pipe comprises atleast one internal sheath similar to that fitted in said monitoringsection, by means of which it is possible to monitor the progression ofthe diffusion of the corrosive gases through the thickness of saidinternal sheath of said pipe.

Other features and advantages of the invention will become apparent onreading the following description of a particular embodiment of theinvention, given by way of nonlimiting indication and with reference tothe appended figures, in which:

FIG. 1 is a partial perspective view of one embodiment of a flexiblepipe that can be used to transport hydrocarbons;

FIG. 2 is a schematic longitudinal cross-sectional view of an internalsheath inspected according to the invention and an ultrasound inspectiontransducer;

FIG. 3 is an example of an echogram obtained during inspection of thesheath using the method of the invention;

FIG. 4 shows the emission spectrum of the ultrasound beam used;

FIG. 5 shows a longitudinal cross section through a pipe monitoringsection, intended to be placed in series with the flexible pipe andinstrumented according to the invention;

FIGS. 6a and 6b illustrate two examples of possible configurations forarrays of elementary transducers possibly suitable for producing theultrasound transducer of the monitoring section according to theinvention; and

FIG. 7 shows an example of a submarine pipe installation in which theinstrumented section is employed according to the invention.

FIG. 1 represents a flexible rough-bore pipe 10 comprising, from theinterior to the exterior: an internal carcass 1 made of profiled metalstrip or wire, with interlocked turns wound with a short pitch, intendedto prevent the pipe from being crushed under external pressure; at leastone internal polymer sheath 2, also called the pressure sheath; astructural assembly of metal armors here comprising a pressure vault 3and tensile armor layers 4; and the external protective sheath 5.

The invention does not depend on the precise configuration of the pipe,it may be implemented in pipes other than pipes of the type illustratedin FIG. 1, for example it may be implemented in a flexible pipe with asmooth internal passage (called smooth-bore pipes) where the passage isformed directly by the internal sheath without a carcass being presenttherein. Thus, the description of the pipe is given merely by way ofindication of one possible embodiment in which the invention may beimplemented.

The internal sheath 2 is made of an extrudable plastic and generally ofa polymer. It may, for example, be made of polyethylene, polyolefin,polyamide or of a fluoropolymer.

According to the invention, the sheath 2 comprises a preset amount of areactive compound that is able to react chemically with the corrosivegases present in the hydrocarbons transported, these gases being liableto diffuse radially through the internal sheath from its inner surfacetowards its outer surface, the reactive compound thus acting as a markermarking the advance of the corrosive gases through the sheath, as willbe explained in greater detail below.

The invention is based on known chemical reactions, used in the field ofmethods for filtering corrosive gases especially as a result of thepresence of H₂S and CO₂, but which have at no point ever been used forthe purpose of monitoring the diffusion of corrosive gases through thethickness of a sheath in the field of flexible pipes for transportinghydrocarbons.

Among the reactive compounds that can be used for the purpose of theinvention, mention may be made of those comprising metal oxides such asZnO, which reacts with H₂S via the following reaction:ZnO+H₂S→ZnS+H₂O.  (1)

Among other reactive compounds of the same type, mention may also bemade, for example, of PbO, CuO, CdO, NiO, SnO₂, MoO₃ and the associatedcarbonate forms.

Thus, according to one embodiment, the internal sheath 2 is manufacturedusing a mixture prepared at the melting point of polyethylene by addingthe reactive compound, for example a metal oxide such as ZnO, beforeextrusion. In this way, the internal polyethylene sheath 2 producedcontains the reactive compound, for example a metal oxide, dispersedthroughout the thickness of the sheath.

The principle on which the invention is based is then detection andlocation of the interface between the reactive compound, for example theZnO, and the reactive compound having reacted with H₂S, for example ZnS,in the sheath via an ultrasonography method, in order to measure theadvance of the diffusion of the corrosive gas through the thickness ofthe sheath.

FIG. 2 shows in detail the internal polyethylene (PE) sheath 2containing the reactive compound, for example ZnO, dispersed through itsthickness. A partial cross section of the sheath 2, which is symmetricabout its longitudinal axis XX′, the latter coinciding with the centralaxis of the pipe, is shown. The pipe is instrumented with an ultrasoundtransducer 10 that is coupled to the sheath 2 so as to emit, radially tothe pipe, an ultrasound beam that passes through the thickness of thesheath and is reflected, to a greater or lesser extent, by obstaclesand, in particular, the outer surface 2 a of the sheath (top of thesheath), the inner surface of the sheath 2 b (bottom of the sheath), andthe interface 6 between the reactive compound, for example ZnO, and thereactive compound having reacted with H₂S, for example ZnS, embodyingthe advance over time of the front of conversion of the ZnO into ZnSthrough the thickness of the sheath, from the inner surface 2 b towardthe outer surface 2 a under the effect of the diffusion of corrosivegases through the thickness of the sheath.

Specifically, when the pipe is in use, corrosive gases such as H₂Spresent in the hydrocarbons transported by the pipe diffuse radiallyfrom the inner surface 2 a toward the outer surface 2 b of the internalsheath 2 and thus form a diffusing-gas layer L1 that gradually extendsfrom the inner surface 2 a through the thickness of the sheath, in whichdiffusing-gas layer the reactive compound, for example ZnO, reacts withthe diffusing corrosive gases via the chemical reaction (1), whereas alayer L2 occupying the residual thickness of the sheath, as yetunreached by the diffusing corrosive gases, extends between the layer L1and the outer surface 2 b of the sheath.

The layer L1 mainly consists of PE and of reactive compound havingreacted with the H₂S, for example ZnS, the latter resulting from thechemical reaction between the ZnO and the diffusing corrosive gases,whereas the layer L2 mainly consists of PE and reactive compound, forexample ZnO, the latter yet to be consumed by the chemical reactionbecause the corrosive gases have yet to diffuse into this layer.

Therefore, the two layers L1 and L2 of the internal sheath have acousticimpedances that differ, respectively, so that it is possible to detectthe interface 6 bounding these two layers by reflecting ultrasonic wavesfrom this interface, and thus to measure the progression of thediffusion of the H₂S through the PE+reactive compound forming thesheath.

The amount of reactive compound that needs to be incorporated into thepolyethylene to manufacture the sheath must therefore be determined inorder to guarantee a sufficiently significant difference between themechanical properties of, on the one hand, the PE+reactive compound, andon the other hand, the PE+reactive compound having reacted with H₂S, soas to allow the interface separating these two layers of material in thesheath to be detected by the ultrasonography method.

The ultrasound signals reflected in the thickness of the internal sheath2 appear on the echogram illustrated in FIG. 3, giving the return signalreceived as a function of time for the embodiment in FIG. 2, where thesheath is made of PE and ZnO and a front of conversion of ZnO to ZnSprogresses, over time, through the thickness of the sheath from itsinner surface 2 b. These reflected ultrasound signals allow obstacles 2a, 2 b and 6 to be detected. Thus the position and amplitude of theechoes from the top of the sheath 2 a, from the bottom of the sheath 2 band from the interface 6 embodying the front of conversion of the ZnO toZnS, are recorded.

It will be noted that the echo from the bottom of the sheath and echofrom the interface have inverse polarities. This feature willadvantageously be used to obtain a certain identification of theinterface echo, even in the presence of a high noise level.

The advance of the front of conversion of the ZnO to ZnS, and thereforethe progression of the diffusion of the gases through the thickness ofthe sheath, may thus be easily measured by calculating the ratio of thetime-of-flight measured for the echo from the interface of the front tothat measured for the echo from the bottom of the sheath. Thus, thismeasurement is temperature independent if the temperature is uniformthrough the thickness of the sheath.

The location of the echoes from the top and bottom of the sheath alsoallow the thickness of the sheath to be checked.

FIG. 2 is simply meant to illustrate the above interface detectionprinciple, and is therefore intentionally schematic, especially withregard to details of how the ultrasound transducer is fitted to thepipe, particulars of which will be provided in the description belowespecially with reference to FIG. 5.

Moreover, it will be noted that it is already known, from patentdocument EP 0 844 429, to use the chemical reaction (1) mentioned aboveto manufacture internal sheaths in the field of flexible pipes, theobject of which reaction is to prevent and, at the very least, limit thepermeability of these sheaths to corrosive fluids such as H₂S. It moreprecisely relates to employing the chemical reaction (1) in the internalsheath to protect the metal layers surrounding this sheath from H₂S byirreversibly neutralizing the corrosive effects of this gas during itsdiffusion through the sheath.

In this respect, it is known that the barrier layer formed by the sheathmanufactured according to the principles of document EP 0 844 429 has alimited lifetime since it loses its effectiveness once all of thereactive compound ZnO added to the material forming the sheath hasreacted with corrosive gases. Therefore, by allowing the position of theinterface 6 of the front of conversion between ZnO and ZnS in thethickness of the sheath to be detected, the invention allows theconsumption of the ZnO in the sheath to be followed in real time, bymeans of which it is possible to accurately determine the remaininglifetime of the sheath forming the anti-H₂S protective barrier. Thus,the invention allows the protection offered by the limited-permeabilitysheath described in document EP 0 844 429 to be combined with theability to monitor, in real time, the effectiveness of the barrier tocorrosive gases provided by this layer, allowing its remaining lifetime,and therefore that of the pipe, to be accurately predicted.

The reader may usefully refer to this document, especially as regardsthe amount of reactive compound to incorporate into the extrudablepolymer in order to ensure the sheath thus produced, depending on theproperties of the sheath (diameter, thickness, polymer) and on the typeof reactive compound used, has the correct permeability to the corrosivegases under given operation conditions.

FIG. 3 shows the spectrum of the ultrasound beam emitted, the spectrumhaving a central frequency (such as defined by standard NFA 09-323)between 2 and 2.5 MHz and preferably equal to 2.25 MHz. Excessively lowfrequencies lead to a low attenuation but to the detriment ofsensitivity, and if the frequency is too high the attenuation is suchthat the signal will not pass right through the sheath if it is verythick.

The pulse width, measured at −20 dB (according to the same standard NFA09-323) is smaller than or equal to 1.5 μs: a short pulse enablescorrect detection near the outer surface and a good depth resolution.The pulse width is preferably larger than 0.5 μs in order to be powerfulenough.

The bandwidth, measured at −6 dB must lie between 1 and 4 MHz andpreferably between 1.5 and 3.5 MHz.

The spectrum of the beam emitted must also comprise a sufficiently largenumber of waves with a low frequency of about 1 MHz the amplitudedifference Δ of which, relative to the central frequency, must not beabove 30 dB. This is because these low-frequency waves promote thepropagation of the emitted beam, of their reduced sensitivity toattenuation in the material of the sheath. The spectrum and the pulsewidth are directly related and depend on the damping of the transducer.

FIG. 5 illustrates an embodiment for implementing the monitoring methodintended to monitor and quantify the progression of the diffusion ofgases through the internal sheath of a flexible pipe in operation. Inthis embodiment, a monitoring section 20 is fitted with an ultrasoundtransducer 10 and this monitoring section 20 is connected in series toan end-fitting for fastening the pipe at the upper end of the latter.

More precisely, the monitoring section 20, which is symmetrical aboutits longitudinal axis XX′, the latter being coincident with the centralaxis of the flexible pipe, comprises two connection flanges 21 and 22,respectively fitted at either end of a cylindrical metal cover 23, aseal 24 being used to seal the assembly. These connection flanges 21 and22 are intended to be brought into contact with pipe end-fittingsfittings or with terminal machinery. The pipe portion incorporated intothe monitoring section is shown by its carcass 11 and its internalsheath 12, the latter having the reactive compound ZnO dispersed throughits thickness. The internal sheath 12 is crimped, at each of its twoends in the section by virtue of parts called crimping cones, 25 and 26,respectively, which slide into and bear against a conical bearingsurface of the corresponding connection flanges, 21 and 22,respectively, and bite into the internal sheath 12, the latter beingsupported, in these locations, by tapered sleeves, respectively 27 and28. A pressure sheath 12′ passing under the sleeves 27 and 28 is placedbetween the carcass 11 and the internal sheath 12. This pressure sheath12′ is itself crimped at each of its ends via two crimping cones, 29 and30, respectively, which bite into the pressure sheath 12′, the latterbeing supported by the carcass 11. In this example, the internal sheath12 filled with reactive compound is separate from the pressure sheath12′, the pressure sheath 12′ being, by definition, the first sheathstarting from the interior. The invention could naturally be applied topipes in which these two sheaths are one and the same sheath.

The flexible pipe portion fitted in the monitoring section 20 is forexample extracted, during manufacture of the flexible pipe to which thesection is intended to be connected in series, from the end of theflexible pipe. This portion extracted from the end of the flexible pipethen has its metal armor layers (pressure vault and tensile armorlayers) and its protective external sheath removed in order to be fittedin the monitoring section as explained above. In this way, themonitoring section 20 has a flexible pipe structure that is, as regardsthe internal sheath to be inspected, identical to that of the flexiblepipe to which it is intended to be connected, thus providing ameasurement environment, for the ultrasound transducer fitted in thesection, that is representative of that of the flexible pipe.

Furthermore, the metal cylindrical cover 23 of the monitoring section 20is equipped with a set of grooves 31 on its inner surface, at theinterface with the outer surface of the internal sheath 12, except inline with the position of the ultrasound transducer 10, so as tosimulate the presence of gaps present in the pressure vault. Therefore,the internal sheath 12 of the monitoring section 20 is placed underconditions that are the same as those that would exist if it weresurrounded by a pressure vault. The set of grooves 31 is connected by alongitudinal groove 32 to an aperture 33 able to be closed by way of aremovable blocking means, the aperture communicating with the exterior,this assembly advantageously forming a circuit for draining gasesdiffusing through the internal sheath 12, via which the pressure towhich the internal sheath in the monitoring section is subjected may becontrolled.

The monitoring section 20 may also be equipped with heating means forcontrolling temperature, not shown, placed substantially around thecylindrical metal cover 23 and allowing the temperature of the flexiblepipe portion, and in particular of the internal sheath 12, fitted in thesection, to be adjusted and controlled. These means may for examplecomprise resistive heaters. They may allow, for example, the pipeportion incorporated into the monitoring section to be placed undertemperature conditions that are more representative of the leastfavorable point in terms of the influence of temperature on thediffusion of gases through the internal sheath, which point is typicallylocated near the well head at the bottom of the flexible pipeinstallation, where the temperature of the transported fluid is at itshighest.

Thus, it is possible to reproduce, in the pipe portion in theinstrumented monitoring section, a measurement environment, especiallyin terms of its high pressure and high temperature, that is similar tothat encountered by the internal sheath of the flexible pipe, theintegrity of which pipe, with respect to the diffusion of corrosivegases through said sheath, is to be inspected by ultrasonography.

Regarding the ultrasound transducer 10, the latter is housed in thecylindrical metal cover 23 and placed a distance h from the outersurface of the internal sheath 12, so as to allow it to detect andobserve the progression of the diffusion of gases through the sheath 12of the pipe portion, according to the principles outlined above. Forexample, the transducer 10 is screwed into the cylindrical metal cover.It is necessary to ensure the steel cover 23 is sufficiently thick, inline with the ultrasound transducer, to withstand the internal pressureused.

Moreover, a passage 34 is provided allowing a first coupling means to beapplied between the front side of the ultrasound transducer and thecylindrical metal cover in which the transducer is housed. In theconventional way, this coupling of the front side of the transducer tothe cover may be achieved by injecting a gel- or oil-based couplingmedium. The dimensions of the passage 34 will be chosen so as to ensuregood transmission of the ultrasound between the transducer and thecylindrical metal cover, while impeding the generation of effects thatcould interfere with the measurement of the ultrasound. The thickness ofthe passage 34, in line with the front side of the transducer 10 is animportant feature as it determines the thickness of the coupling-mediumfilm that the ultrasound beam has to pass through. In practice, thispassage thickness must advantageously be much smaller than half thewavelength used. By way of example, for a transducer central frequencyof 2 MHz, the propagation speed of ultrasound in steel being equal toabout 6000 m/s, the wavelength in steel is λ=c/N, c being thepropagation speed and N being the frequency, i.e. substantially 3 mm. Itwill therefore, for example, possibly be chosen to couple the front sideof the transducer and the cylindrical metal cover with a film of oilhaving a thickness of about one tenth of a millimeter. As a variant,this coupling could also be obtained by positioning a thin washer madeof an elastomer at the bottom of the hole in which the ultrasoundtransducer is housed.

A second coupling means is also required between the cylindrical metalcover and the outer surface of the internal ZnO-filled sheath 12. Thissecond coupling means is advantageously just the internal pressuregenerated by the fluid transported by the flexible pipe portion, thispressure being about one hundred bars. Specifically, provided that thecylindrical metal cover has been suitably machined to have a smoothsurface finish, the mechanical coupling, under pressure, between theinner surface of the cylindrical metal cover and the outer surface ofthe internal sheath is alone enough to obtain a clear interface andtherefore good coupling, allowing the ultrasonography measurement to becarried out. Thus, the invention advantageously makes use of thehigh-pressure conditions to which the flexible pipe is subjected in useto achieve this second coupling between the outer surface of theinternal sheath and the cylindrical metal cover, which coupling isnecessary if the ultrasound measurement is to be carried out correctly.

Thus, by virtue of these means for coupling the front side of theultrasound transducer 10 to the outer surface of the internal sheath 12,the ultrasound wave generated by the transducer passes, via thesecoupling means, through the thickness of steel remaining in thecylindrical metal cover, in line with the transducer, before penetratinginto the internal ZnO-filled sheath. On return, after reflection fromthe bottom of the internal sheath, the ultrasound wave takes the samepath but in the opposite direction.

Furthermore, to obtain an echogram, such as that shown in FIG. 3, thatcan be used with a view to monitoring the diffusion of corrosive gasesthrough the sheath, it is advantageous to carefully define the distanceh at which the front side of the transducer 10 is located relative tothe internal sheath of the pipe portion fitted in the monitoringsection. In particular, the distance h must be chosen such that therebound echo of the ultrasound echoed from the front side of thetransducer 10 is sensed after the echo from the sheath bottom, so as notto introduce noise over the echoes that are of interest, especially theecho from the top of the sheath, the echo from the ZnO/ZnS frontinterface and the echo from the bottom of the sheath. To do this, thedistance h is defined in the following way:

$h \geq {e \times {\frac{V\; 1}{V\; 2}.}}$

Where:

e is the thickness of the internal sheath 12;

V1 is the propagation speed of the ultrasound in the cylindrical metalcover; and

V2 is the propagation speed of the ultrasound in the internal sheath.

In practice, V1 is about 6000 m/s and V2 is about 2000 m/s (case of PE),so that the thickness h of steel remaining between the front side of thetransducer and the outer surface of the internal sheath must be largerthan at least three times the thickness of the sheath in order to meetthe aforementioned condition regarding propagation of the waves.

Furthermore, if it is desired to extend the measurement field beyond theinternal sheath, as far as the carcass, the thickness of the sacrificialsheath 12′ is taken into account so that the rebound echoes on the frontside of the transducer arrive after the echo generated by the carcass,so as to obtain a “clean” echogram as far as the carcass.

Moreover, it is desirable to focus the ultrasound wave emitted in orderto improve the signal-to-noise ratio of the response of the transducer.The focal point is the point where the ultrasound wave emitted by thetransducer exhibits maximum intensity. The intensity gradually decreaseswith distance from the focal point. The −6 dB focal spot is, bydefinition, the volume surrounding the focal point in which theintensity of the ultrasound remains at least higher than 50% of theintensity at the focal point. A known means for measuring the size ofthe focal spot consists in immerging the transducer in a tank filledwith water and moving a small metal ball into the field of thetransducer, the ball reflecting the ultrasound. For each relativeposition of the ball and transducer, the amplitude of the ultrasoundecho reflected from the ball is recorded, this amplitude beingproportional to the intensity of the wave at the point occupied by theball. By moving the ball along 3 axes so as to map out the entire volumeoccupied by the ultrasound beam, the position of the focal point and theedges of the −6 dB focal spot can be determined.

Common focused transducers have a structure that is symmetric about theaxis of the acoustic beam. The −6 dB focal spot of such sensors is alsosubstantially symmetric about the axis of the acoustic beam, andgenerally takes the shape of an ellipsoid of revolution. The width ofthe focal spot, also called the diameter of the focal spot, is measuredin the focal plane, i.e. in the plane perpendicular to the axis of theacoustic beam and passing through the focal point. According to theinvention, the focal spot is advantageously about a few millimeters inwidth and ideally is chosen to be, at most, 2 mm to 5 mm in width. Thelength of the focal spot is measured along the axis of the acousticbeam. According to the invention the length of the focal spot is chosento cover the entire thickness of the internal sheath 12. In fact, it ispreferable to choose a spot length much larger than the thickness of thesheath, and to focus the spot towards or beyond the bottom of the sheathso as to easily cover the entire sheath and to reduce sensitivity tovariations in the distance between the transducer and the sheath. Theacoustic focus coefficient will be chosen so as to obtain an optimalsignal-to-noise ratio for the envisioned application of monitoring thediffusion of corrosive gases through the sheath.

A focus may be obtained using ultrasound lenses; appropriately shapedmirrors; by shaping the piezoelectric transducer, especially if it is apiezocomposite ultrasound transducer; or using an electronicallyphase-shifted array of elementary transducers (i.e. a phased array).

FIGS. 6a and 6b illustrate two possible configurations for such phasedarrays, which may be suitable for the ultrasonic transducer 10, i.e. atwo-dimensional square pattern and a segmented two-dimensional annularpattern.

FIG. 7 shows an exemplary configuration for a monitoring section 20,instrumented as explained above, fitted to a flexible pipe installation40 of the type used in a bottom-to-surface connection between asubmarine piece of equipment, such as, for example, a well head, and asurface unit, which may consist of a floating platform or ship.According to this embodiment, the submarine pipe installation 40comprises a vertical flexible pipe 41 the lower end of which isconnected to the well head at the bottom of the sea and the upper end ofwhich opens onto the sea surface; a linking pipe 42, joining the upperend of the vertical flexible pipe 41 and the surface unit, and a by-passpipe 43 between the upper end of the vertical flexible pipe and thesurface unit, in parallel with the linking pipe 42. The monitoringsection 20 equipped with the ultrasound transducer 10 for implementingthe method of the invention is connected in series with the linking pipe42 via its two connection flanges 21 and 22 fastened to correspondingfastening end-fittings of the linking pipe. Thus, the ultrasoundtransducer is easily accessible, which, in particular, makes it easierto connect it to a signal processing unit. Furthermore, valves 42 a, 43a and 44 a allow the flow of fluid in the fluid circuit formed by thevarious pipes of the installation to be controlled. In particular, thelinking pipe portion incorporating the monitoring section 20 may beisolated from the fluid circuit by closing the valves 42 a and 44 a, thevalve 43 a being opened to allow the fluid to flow through the by-passpipe 43. In this way, work may be carried out on the monitoring section20 in order to maintain and/or test the operation of the transducer.

As a variant, the ultrasound transducer may also be incorporateddirectly into one of the fastening end-fittings of the flexible pipe.Specifically, it is possible to house the ultrasound transducer underthe armor layers behind the end-fitting, on the flexible pipe side. Itis also possible to lengthen the end-fitting on the flange side so as toextend the internal sheath beyond the anchoring zone of the armorlayers, and to place the ultrasound transducer in line with the extendedzone.

As regards rigid pipes comprising an internal liner, it is possible toinstrument them by directly placing the ultrasound transducer on theexterior of the metal tube surrounding the liner, said metal tubepossibly then being equivalent to the cylindrical metal cover (23) inFIG. 5.

What is claimed is:
 1. A monitoring section for being attached to atubular pipe, wherein the pipe transmits hydrocarbons containingcorrosive gases; the monitoring section comprising, from a radialinterior to an exterior of the monitoring section, an internal sheath onthe interior of the section, a cylindrical metal cover fitted around theinternal sheath, and an integrated ultrasonic transducer configured andoperable to determine the position of the interface between the firstand second layers using ultrasound to measure, in real time, progressionof the diffusion of the corrosive gases through the thickness of theinternal sheath by determining the location of the interface; theinternal sheath having an inner surface defining a path through andalong which the hydrocarbons pass and having a radially outer surfaceand a thickness between the inner and outer surfaces and the sheathpermitting diffusion of the corrosive gases radially through the sheath;the internal sheath being comprised of a polymer in whichreactive-compound elements are dispersed throughout the thickness of thesheath between the inner and outer surfaces, wherein thereactive-compound elements are operative to react with corrosive gasesfrom the hydrocarbons which diffuse through the thickness of the sheathto neutralize the corrosive gases, the polymer sheath being soconstructed that the corrosive gases are liable to diffuse radiallythrough the thickness from the inner surface of the sheath to the outersurface of the sheath, such that as the corrosive gases diffuse radiallyoutwardly, they react with the reactive-compound elements to form afirst layer from the inner surface of the sheath radially outwardly inwhich the reactive-compound elements have reacted with the corrosivegases, and the first layer gradually moving radially outwardly throughthe thickness of the sheath toward the outer surface, and the sheathfurther comprising a second layer occupying the residual thickness ofthe sheath radially outwardly from the first layer, wherein thereactive-compound elements in the second layer of the sheath have notreacted with corrosive gases, such that an interface is formed betweenthe first and the second layers, which interface moves graduallyradially outward through the thickness of the sheath as corrosive gasesreact with the reactive-compound elements.
 2. The monitoring section asclaimed in claim 1, wherein the cylindrical metal cover has a thicknessbetween a front side of the transducer which is an outward side of thecylindrical metal cover, such that the residual thickness of thecylindrical metal cover between the front side of the transducer and theouter surface of the internal sheath is larger than at least three timesthe radial thickness of the sheath.
 3. The monitoring section as claimedin claim 2, further comprising a first coupler for coupling the frontside of the ultrasonic transducer to the cylindrical metal cover of thetransducer and a second coupler for coupling the outer surface of theinternal sheath to the cylindrical metal cover.
 4. The monitoringsection as claimed in claim 3, wherein the second coupler comprises apressurized device for mechanically coupling the outer surface of theinternal sheath to an inner surface of the cylindrical metal cover. 5.The monitoring section as claimed in claim 2, wherein the cylindricalmetal cover comprises a circuit for draining gases diffusing through theinternal sheath, the draining circuit comprising a set of grooves formedon an inner surface of the cover and at an interface with an outersurface of the internal sheath, the set of grooves being connected to anaperture configured and operable to be closed by a stopper, and the setof grooves communicating with the exterior of the monitoring section,wherein pressure to which the internal sheath is subjected iscontrolled.
 6. The monitoring section as claimed in claim 1, furthercomprising connecting flanges respectively fitted at either end of thecylindrical metal cover and crimping cones on the flanges configured andoperable to bear against the internal sheath.
 7. A pipe for transportinghydrocarbons containing corrosive gases, the pipe comprising at leastone monitoring section as claimed in claim 1; the pipe comprising atleast one internal sheath with the characteristics of the internalsheath in the monitoring section and the internal sheath in the pipe isconfigured to be monitored to monitor the progression of the diffusionof the corrosive gases through the thickness of the internal sheath ofthe pipe.